Symmetry, Integrability and Geometry: Methods and Applications SIGMA 8 (2012), 032, 15 pages On the Relations between Gravity and BF Theories? Laurent FREIDEL y and Simone SPEZIALE z y Perimeter Institute, 31 Caroline St N, Waterloo ON, N2L 2Y5, Canada E-mail: [email protected] z Centre de Physique Th´eorique,CNRS-UMR 7332, Luminy Case 907, 13288 Marseille, France E-mail: [email protected] Received January 23, 2012, in final form May 18, 2012; Published online May 26, 2012 http://dx.doi.org/10.3842/SIGMA.2012.032 Abstract. We review, in the light of recent developments, the existing relations between gravity and topological BF theories at the classical level. We include the Plebanski action in both self-dual and non-chiral formulations, their generalizations, and the MacDowell{ Mansouri action. Key words: Plebanski action; MacDowell{Mansouri action; BF gravity; TQFT; modified theories of gravity 2010 Mathematics Subject Classification: 83C45 1 Introduction It is an intriguing fact that general relativity can be given a polynomial action principle which relates it to topological BF theory. The fundamental variable is a connection, and the metric is a derived quantity. For BF theory alone, a Lagrange multiplier, B, enforces the curvature of the connection to be constant. This type of theories require no metric to be formulated, and posses no local degrees of freedom. The relation with general relativity is established through the extraction of metric degrees of freedom from the fundamental fields. There are mainly two ways of achieving this that have appeared in the literature, based on original work by Plebanski [77] and by MacDowell and Mansouri [66]. The two ways differ in the choice of gauge group, and in the way the metric is recovered { from either the B field or the connection. In this review, we will go through the basic aspects of these two approaches, as well as some recent developments. We restrict our attention to four spacetime dimensions. For the connection between BF theory and gravity in three dimensions, see [22, 30, 98], and [47] in dimensions higher than four. 2 BF theory Let us begin with some general aspects of BF theory [22]. We consider a principal bundle over the spacetime manifold M, with local group a Lie group G and connection !, with curvature F (!), and the following action, Z λ S(B; !) = tr B ^ F − tr B ^ B; (1) 2 where B is a 2-form in the adjoint representation of G and tr denotes the scalar product in the algebra. λ is a dimensionless constant. Notice that the action is well-defined without any reference to a metric structure on M. The field equations are F (!) = λB; d!B = 0: ?This paper is a contribution to the Special Issue \Loop Quantum Gravity and Cosmology". The full collection is available at http://www.emis.de/journals/SIGMA/LQGC.html 2 L. Freidel and S. Speziale The action is invariant under local gauge transformations, G G δα B = [B; α]; δα ! = d!α; and under the following shift symmetry, S S δη B = d!η; δη ! = λη: (2) The total symmetry group is thus the semi-direct product of these two groups, and it is the same for all values of Λ. Overall, there are N gauge parameters in the algebra-valued scalars αi, i and 4N in the algebra-valued 1-forms ηµ. The system has so much symmetry that all solutions are locally gauge equivalent, and local degrees of freedom are absent. This is most directly established in the canonical formalism. i a 1 abc The phase space of (1) is spanned by the 6N variables (!a; Πi := 2 Bibc), where a = 1; 2; 3 are space indices. The action takes the form Z a i i i a S = Πi !_ a + !0Gi + B0aCi ; and the (fully constrained) dynamics is given by the N Gauss constraints Gi and 3N \Hamilto- a nian" constraints Ci , 1 G = D Πa = 0; Ca = abcF (!) − λΠa = 0: (3) i a i i 2 ibc i a As a consequence of the Bianchi identity, DaCi ≡ 0, thus there are only 3N independent constraints. The algebra of constraints is fC(N);C(N 0)g = 0; fG(α);G(α0)g = G([α; α0]); fC(N);G(α)g = C([N; α]); R i a R i where C(N) = NaCi , G(α) = α Gi. We see that they form a first class system, thus they reduce the phase space by 2×3N = 6N dimensions, leaving a zero-dimensional space of solutions. Hamiltonian systems with linearly dependent constraints are called reducible [22]. The symmetry group also includes the action of diffeomorphisms, which manifestly leaves (1) invariant. Diffeomorphisms are expressed as special combinations of gauge and shift transfor- mations [26, 48]. Recalling that the Lie derivative of a 1-form verifies Lξ! = iξd! + d(iξ!), where iξ denotes the inner product with the vector ξ, it is easily checked that δDB = dG B + δS B + i (d B); δD! = dG ! + δS ! + i F: ξ iξ! iξB ξ ! ξ iξ! iξB ξ Accordingly, the canonical generators of hypersurface deformations can be expressed as linear combinations of (3). In spite of the lack of local dynamics, BF theory has many interesting applications. The subject of this review is the classical relation to general relativity, but the action has been used also in connection with Yang{Mills theory (e.g. [31]). Concerning the quantum theory, we refer the reader to [22] for the continuum path integral quantization and its relation to the measure of flat connections and Reidemeister torsion, and to [10, 13, 25, 75, 99] for the discrete path integral and its relation to spin foam models for quantum gravity. 3 Self-dual Plebanski action A way to relate BF theory to general relativity is to work with the local gauge group SU(2), seen as a chiral subgroup of the Lorentz group. The fundamental fields are (Bi;!i), with On the Relations between Gravity and BF Theories 3 i = 1; 2; 3 indices in the adjoint representation. Then one can write a densitized symmetric tensor as [28, 94] 1 pjgj g = αβγδBi Bj Bk : (4) µν 12 ijk µα βγ δν A theorem by Urbantke states that if the bilinear density mij ≡ Bi ^Bj is invertible as a matrix in the ij indices, then also the Urbantke metric (4) is invertible, and furthermore B is self-dual i p ρσ i (for det m > 0, else antiself-dual) with respect to it, that is Bµν = ±1=(2 jgj)µν Bρσ. See e.g. [45] for a recent proof. Therefore, provided the B field is not degenerate in the above sense, we have a natural way to introduce an invertible metric as a composite object. For B real, compatible with the case of Euclidean signature, one automatically obtains a real metric with positive determinant1. For B complex, as required by the chiral splitting of the Lorentz group with Lorentzian signature, one obtains a complex metric with negative determinant, and additional reality conditions need to be given [28]. To deal with this formalism, it is useful to introduce a tetrad eI for the metric (4), I J gµν = eµeν ηIJ ; (5) i i I J 0 i p i j k and define the Plebanski 2-forms Σ ≡ PIJ e ^ e = e ^ e + σ/2 jke ^ e , where σ = ± a is the spacetime signature, = ± and P(±)IJ are the projectors on the left- and right-handed su(2) subalgebras, according to the isomorphism so(3; σ) =∼ su(2) ⊕ su(2), and we have fixed the time gauge in the internal indices. These forms have the property that the left-handed part (resp. right-handed) is simultaneously self-dual (resp. antiself-dual) in the spacetime indices with a respect to e. The set Σ provides an orthogonal basis for the 6-dimensional space of 2-forms. Combining this fact with Urbantke's theorem, we conclude that a generic, albeit non-degenerate, B field can be written as a linear combination of Plebanski's 2-forms. This can be conveniently parametrized as follows, i i a i B(±) = ηbaΣ(±); det ba = 1; η = ±; (6) i where the unimodularity of ba can always be we assumed, as we did here, thanks to the conformal invariance of the notion of self-duality. The parametrization (6) helps to appreciate the relation between SU(2) BF theory and general relativity: it shows how the B field can equivalently be parametrized in terms of a metric gµν together with the eight components of the unimodular i i \internal triad" ba, and a sign. From the \triad" ba, one can define the unimodular \internal i j metric" qab := babbδij, given by an SL(3)-rotated form of the identity. Comparing (4) with (5), we see that the B field plays the role of a chiral \cubic root" of the metric, with the SL(3) group i 2 formed by the ba replacing the internal Lorentz group in (5)[90] . The relation with general relativity can now be easily established: It suffices to impose the i condition that ba 2 SO(3), or equivalently that qab = δab, and the BF action (1) immediately reduces to (η times) the Einstein{Cartan action in self-dual variables, with cosmological constant Λ = −3ηλ. The required condition on b can equivalently be written as 1 Bi ^ Bj = δijB ^ Bk; (7) 3 k a form known as Plebanski or metricity constraints. Using a traceless symmetric scalar 'ij as a Lagrange multiplier, the constraints can be included in the action, Z λ 1 S(B; !) = B ^ F i(!) − B ^ Bi + ' Bi ^ Bj: (8) i 2 i 2 ij 1Notice that this includes the case of Kleinian signature (+ + −−).